Influenza virus is also known as IFV for short. Influenza virus is one of the earliest studied viruses and is received much concern due to its severe damage, which exhibits rapid outbreak and spread, and thus leads to local or global epidemics and disastrous consequences in a short period of time. There were 5 influenza outbreaks in the last century. In 1918, the Spanish Flu Pandemic (H1N1) had taken the lives of 20 million people. Later happened in 1957 was the Asian Flu, an influenza virus A H2N2 subtype derived from gene reassortment of human influenza viruses with avian influenza viruses, wherein three gene fragments, namely HA, NA, and PB1, came from avian influenza viruses, and other gene fragments came from human influenza viruses at the moment. The Hong Kong Flu, which was first found in Hong Kong and later spread to Japan and the United States, etc., killed many lives in 1968. Hong Kong Flu was influenza virus H3N3 subtype, which was also derived from gene reassortment of human influenza viruses with avian influenza viruses. The Russian Flu in 1977, derived from H1N1 subtype reassortment based on influenza virus A H5N1 subtype, spread relatively slow and mainly infected the youth and poultries. In 1998-1999, Hong Kong suffered again from influenza virus A H5N1 subtype epidemic. Furthermore, many lives of children and the old are taken by smaller epidemics every year, which results in serious social problems. Besides of epidemics in human being, influenza virus may also cause infection in animals, such as bird flu or swine flu etc., leading to tremendous economic loss in breeding industry. Starting from Southeast Asia in the winter of 2004, the H5N1 high pathogenic avian influenza resulted in grievous loss in many Asian and European countries and regions, and led to more than 100 cases of infections with more than 80 deaths all over the world. In 2007, the Mexican swine influenza gave rise to global attention. The frequent influenza epidemics are mainly because of the frequent variation of influenza virus, which makes the prevention and the control of influenza a great challenge to human beings.
Influenza viruses belong to Orthomyxoviridae, and are segmented minus-strand RNA viruses. According to different antigenicity of nucleocapsid protein (NP) and matrix protein (M), influenza viruses are divided into A, B and C types. The influenza viruses have a spherical shape with a diameter of 80-120 nm, whereas newly isolated strains are mostly filament-shaped with 400 nm in length. Influenza viruses comprise, from outside to inside, three parts of the envelope, the matrix protein and the core. The envelope is a phospholipid bilayer membrane coated outside of the matrix protein. The membrane is derived from cell membrane of the host, mature influenza virus buds from the host cell and separates from the cell after being coated outside with host cell membrane, to infect the next target. Except for phospholipid molecule, the envelope contains two important glycoproteins, namely hemagglutinin and neuraminidase, which extrude outside of the virus with 10-40 nm in length and are known as spikes. Usually there are 500 hemagglutinin spikes and 100 neuraminidase spikes on the surface of one influenza virus. In influenza virus A, the antigenicity of hemagglutinin and neuraminidase varies, which is a standard for distinction of viral strain subtypes.
Among three influenza viruses infecting human beings, influenza virus A has a very strong variability, and influenza virus B takes the second place; whereas the antigenicity of influenza virus C is very stable. As the most frequently varying type, influenza virus A intends to have a great antigenicity variation in every dozen years, which will result in a new virus strain. This variation is called antigenic shift, i.e., qualitative change of the antigen. Small changes also take place in influenza virus A by the form of point mutation in antigen amino acid sequence, which is called antigenic drift, i.e., quantitative change of the antigen. Antigenic shift may be a simultaneous change of both hemagglutinin antigen and neuraminidase antigen, which is called family variation; or, it may be a variation of hemagglutinin antigen only and neuraminidase antigen does not change or changes only a little, which is called subtype variation.
The influenza virus genome consists of 8 negative single stranded RNA fragments encoding 10 virus proteins, among which 8 are components for virus particles (HA, NA, NP, M1, M2, PB1, PB2 and PA), and the other two are RNA fragments with the smallest molecular weight encoding two non-structural proteins, namely, NS1 and NS2. NS1 relates to cytoplasm inclusion body. However, it is at present not very clear about functions of NS1 and NS2.
There are two glycoprotein spikes with different shapes and the same length of 10-20 nm on the surface of influenza virus A particles, respectively, hemagglutinin (HA) and neuraminidase (NA), both possessing antigenicity. The ratio of HA to NA is 4:1 to 5:1. There is also a transmembrane protein (M2 channel protein) of smaller amount on the surface of virus particles, the ratio of M2 to HA being 1:10-100. Located under the bilayer lipid envelope of virus particle is the matrix protein M1, whose content is the largest in the virus particle and which constitutes the main structure of the virus envelope. A complex of nucleic export protein (NEP, non-structural protein2, NS2) and viral ribonucleoprotein (RNP) is located inside of M1 and links thereto. The studies of the invention mainly focus on three genes of M, NP and PB2.
According to antigenicity of HA and NA, influenza virus A may be classified into different subtypes, among which there are 16 HAs (H1-16) and 9 NAs (N1-9). Avian influenza virus includes all subtypes, whereas human influenza virus includes only H1-H3 and N1-N2, and swine influenza virus includes only H1, H3, and N1-N2 subtypes.
As for genome of influenza virus, the genome of influenza virus A contains 8 RNA fragments (see FIG. 1), and at the 5′ and 3′ terminal of every fragment there is one non-encoding region, which is relatively conserved among all fragments and encodes one or more virus proteins. The biggest three RNA fragments encode polymerase, the 4th RNA fragment encodes HA protein, the 5th encodes NP protein, the 6th encodes NA and NB proteins (NB protein helps to infect), and the 7th encodes M1 protein (whose stop codon overlaps with the start codon of M2, to form a “stop-start” UAAUG sequence). The 8th fragment encodes NS1 as well as the NEP/NS2 protein through splice.
Due to the high variability of influenza, it is especially important to choose a silent target gene.
The matrix protein (MA) is a non-glycosylated structural protein, including two types, namely M1 and M2, and its structure is stable during the evolution of the virus. M1 regulates the export of ribonucleoprotein, and inhibits virus gene transcription as well as has the key effect thereof in virus budding and so on. M2 is a transmembrane protein with a conserved structure. M2 regulates pH gradients of transmembrane transportion tunnel of Golgi apparatus and stabilizes the HA protein. M1 consists of 252 amino acid residues with a molecular weight of about 26 kDa, and has no glycosylation site at the polypeptide chain. Except for being a structural protein, M1 participates in the regulation of virus transcription and the material delivery between the nucleus and cytoplasm of infected cells. M2 gene comprises two parts of 14-39 and 728-995, whose initial transcription product is cleaved of the 40-727 introns and spliced into mature mRNA, then translated into M2 protein. M2 protein is an intact membrane protein with 97 amino acids that is expressed massively at the surface of infected cells, including extracellular N-terminus (23 amino acid residues), transmembrane region (25-43 amino acid residues) and intracellular C-terminus (54 amino acid residues). M2 protein is a relatively smaller viral component, whose bioactive form is an oligo-tetramer. M2 protein includes 3 cysteine residues, Cys217, Cys219 and Cys250. Most M2 form an M2-dimer via a disulfide bond between Cys217 and Cys219, then a homotetramer via non-covalent bond linkage wherein Cys250 is acylated. Site specific mutation proves that although it is not a necessity for tetramer assembly, the disulfide bond can stabilize the M2 tetramer structure. If Cys217 and Cys219 are lacked, mutated tetramer can only play a role of cross-linking, and the virus may not survive or have weakened toxicity. M2 protein plays an important role in virus replication, and is also a target for anti-virus drugs.
Nucleoprotein (NP), a main protein component of virus nucleocapsid, is a monomer phosphorylated polypetide with a molecular weight of about 60 kDa and is type-specific, thus becoming a factor for the specified host selection. Encoded by vRNA fragment 5, nucleoprotein molecule consists of 498 amino acid residues and is rich in arginines. The amino acid sequence is conserved among subtypes and among various strains of one subtype, and provides basis for the evolution and classification of influenza virus molecules. Nucleoprotein, a main component of virus nucleocapsid, is type-specific and a chief antigen recognized by cytotoxic lymphocyte. It also plays a part in viral gene transcription and replication.
Structure and function of PB2 subunit: the PB2 subunit is another component of polymerase trimer and has many functions. On virus infection, the PB2 subunit determines host specificity of the virus by passing between cell nucleus and cytoplasm, the crucial site thereof being amino acid residue at position 702 [5]. The PB2 subunit also relates to viral pathogenicity wherein amino acid residue at position 627 plays a vital role. The D701N mutation can break the salt bridge between NSL and Arg753 of PB2, and potentially change the aggregation of 3 polymerase subunits, thus influences interspecies transmission of influenza viruses. When viral genome transcribes, the PB2 subunit can bind to cap structure. However, there is a controversy about the cap bonding site. Honda et al. (1999) believe that it relates to amino acid residues at position 242 and 252, Li et al. (2001) consider that it relates to amino acids in the region of 533-564, and Fechter et al. (2003) think that amino acid residues at position 363 and 404 are crucial ones for binding with cap structure, and 2 aromatic amino acids are extremely conserved in influenza virus A and B, which is of profound significance on the study of the function of influenza virus PB2 subunits of different subtypes. PB2 N-terminus links to PB1 C-terminus, wherein the amino acid residue at position 249 of PB2 plays a vital role. PB2 subunit recognizes the 5′ terminus of vRNA promoter, specifically binds thereto, and cleaves the host cell mRNA as an endonuclease to create transcription primers, and thus starts viral genome transcription process which depends on the primer. However, the conclusion is still in dispute and needs more studies. PB2 is found to exist in the mitochondria of both transfected cells and virus infected cells. The positioning signal of PB2 in cell nucleus lies between 449-495 amino acid residues of PB2, and the target positioning signal is at the N-terminus thereof in the mitochondria. 2 conserved leucine residues play a vital role during this positioning process.
Among the several proteins of influenza virus, HA, NA, NP, M1 and M2 proteins work as virus structural proteins and participate in the assembly of virus particle, and play an important role in the maintenance of virus morphology, the assembly of capsomers, as well as the process of adsorption, invasion and release etc. of virus. NS1, NS2, PB1, PB2 and PA are non-structural proteins that do not directly participate in the assembly of virus capsomers, instead they exhibit important regulation effect in the virus replication cycle. Therefore, with thorough consideration of elements all around, the present invention is focused on conserved and important functional genes, such as M, NP and PB2.
Antisense nucleic acid includes antisense RNA and antisense DNA, and is characterized in its convenient synthesis, sequences being easily designed, modified, and high selectivity and affinity and so on. As a new anti-virus and anti-tumor drug, antisense nucleic acid raises a revolution in pharmacology field, namely, carries out the reaction after the binding of new drug receptor mRNA via new receptor binding mode (Watson-Crick hybridization), comprising (1) RNase H mediated degradation of target RNA; (2) inhibition of DNA replication and transcription as well as post-transcriptional processing and translation, etc. The antisense oligonucleotides (ODNs) treatment may exhibit higher specificity than traditional drug therapeutic tools. In the 30 years since late 1970s, antisense nucleic acid drugs have walked out of labs into practical clinical application. Antisense therapeutics has been paid more attention to especially after the first antisense nucleic acid drug, Fomivirsen, is approved to be listed in the market by FDA.
On the basis of base-pairing principles, antisense nucleic acid may participate in the regulation of related gene expression via base pairing with target RNA. The possible mode of regulation may be as follows: I. antisense RNA binds to virus mRNA, to form complementary double strand and block the binding of ribosome to virus mRNA, and thus inhibits process of virus mRNA being translated into protein; II. A triple helix nucleic acid can be formed by antisense DNA and the target gene, which regulates gene transcription by acting on transcriptons, enhancers and promoter regions controlling gene transcription; III. The binding of antisense nucleic acid to virus mRNA stops the delivery of mRNA to cytoplasm; IV. The binding of antisense nucleic acid to virus mRNA makes mRNA more easily to be recognized and degraded by nuclease, and thus greatly shortens mRNA half-lives. The above four pathways may exhibit as the inhibition or regulation on the virus gene expression, and this regulation is highly specific.
The antisense nucleic acid recognizes target gene on the basis of base-pairing principle. Theoretically, taking animal cells for an example, whose chromosomes have about several billions of base pairs, if the 4 bases (A, G, C and T) are roughly of the same amount and randomly distributed all over the gene, then it is not likely for antisense nucleic acids with more than 17 bases to hybridize with non-target gene, according to statistics principle. Therefore, it could be said that the binding of antisense nucleic acid molecules with more than 17 bases to target genes is unique, which results in high specificity of antisense nucleic acids.
Studies show that one gene copy will generate 200-300 mRNAs, which will then be translated into 100 thousand protein molecules with bioactivity. Traditional drugs mainly react with several sites of a certain domain in a bioactive protein molecule. However, because of the extremely complex protein structures and the various space structures of active proteins in an organism, it is actually not easy to get an ideal effect by controlling the dynamic and integral function of target molecules via several limited kinds of acting sites of the traditional drugs and the limitation of the traditional drugs is obvious. The mRNA can be translated into tens to hundreds of proteins, and antisense nucleic acids regulate the target gene directly at mRNA level, which equals to amply the effect of the traditional drugs by tens to hundreds of times, from which it can be seen that the regulation of the antisense nucleic acids is extremely economic and reasonable.
Toxicology researches show that, antisense nucleic acids exhibit low toxicity in vivo. In spite of longer or shorter retention time in vivo, antisense nucleic acids are all eliminated by degradation, avoiding risks of foreign gene integration into host chromosomes, such as what happened in trans-genetic therapy. Compared to traditional drugs, antisense nucleic acid drugs show advantages of high specificity and efficiency as well as low toxic and side effect and so on, and exhibit good application value at various aspects, such as tumor inhibition and virus replication resistance, etc. Nowadays there are multiple antisense nucleic acid drugs in American and European markets, and 30 more at preclinical develop phase or experimental phase I, II or III after develop.
Because of a plenty of exonucleases existed in animals, antisense nucleic acids without chemical modification are easily to be degraded and lose activities. At present, there are many approaches for chemical modification of antisense nucleic acids, such as sulfo-modification and 2′-methoxy modification, etc. Being most thoroughly studied at present, sulfo-modified drugs can effectively resist degradation of nuclease and promote the activity of nuclease Rase H, said modification method having been successfully used in clinical antisense nucleic acid drugs. However, these are only the first generation of antisense nucleic acid modification method, and new modification approaches and methods are developed with technology development, leading to second and third generations of antisense nucleic acid modification, in which peptide framework modification is most eye-catching.
Peptide nucleic acids (PNAs), a brand new DNA analog with neutral amido bond as framework, may target DNA big grooves in specific sequence. The structural unit of PNA framework is N (2-aminoethyl)-glycine, and base groups link to the amino N atom of main framework by a methylene carbonyl group. PNA is the second generation product of the antisense nucleic acid.
As a natural cationic polymer, chitosan (cs) is not only easy to bind with genetic materials such as genes (nucleic acids) to form nano-particles, but also has advantages such as non-toxicity, easily availability, biodegradability, stability, biocompatibility, gastrointestinal environment (pH, nuclease) destruction resistibility, strong bioadhesiveness, and being able to promote the permeating and adsorbing of the drugs, etc., therefore becoming a good carrier for oral-taken gene drugs.